The present invention relates to generally to an efficient amplifier of millimeter wavelengths, and more particularly to a gyromagnetron amplifier which operates at a relatively low operating beam voltage and a relatively low magnetic field.
Gyrotrons (cyclotron resonance masers) have proved to be efficient high power devices in the generation and amplification of radiation at millimeter wavelengths. Most gyrotron oscillators and amplifiers operate at the fundamental cyclotron harmonic and, with a few exceptions, at the second harmonic. For a gyrotron to operate at 100 GHz at the fundamental cyclotron harmonic, a magnetic field in excess of 35 kG would be required. Such a high magnetic field can only be provided by a superconducting magnet, and is regarded as undesirable from practical considerations.
It is known that for electron devices which utilize the cyclotron resonance, the required magnetic field may be reduced by a factor of L if the device is operated at the Lth cyclotron harmonic frequency. In this regard, see the article "High Frequency Electron Discharge Device", by J. Feinstein and H. R. Jory, U.S. Pat. No. 3,457,450; and the article "Theory of Electron Cyclotron Maser Interaction in A Cavity At The Harmonic Frequencies," by K. R. Chu, Phys. Fluids, Vol. 21, pages 2354-2364, 1978. In an experiment along this line by Destler et al. described in the article "High Power Microwave Wave Generation From A Rotating E Layer In A Magnetron-type Waveguide," Applied Physics Letters, Vol. 38, pages 570-572, 1981, and the article "Intense Microwave Generations From a Non-Neutral Rotating E-Layer," Journal of Applied Physics, 52, pages 2740-2749, 1981, a waveguide wall was utilized with 12 corrugations therein so that the circuit resembled the outer boundary of a conventional or relativistic magnetron. Such a modification permitted operation at the 12th cyclotron harmonic where a sharp increase in the output power on the order of 250 MW was obtained.
It can be seen that it is extremely desirable to utilize a high cyclotron harmonic frequency in the gyrotron device in order to significantly reduce the required magnetic field. The most efficient mode of operation at the Lth cyclotron harmonic is the circular TE.sub.L1 mode. For a high harmonic number L, the electric field for the TE.sub.L1 mode is highly concentrated toward the wall of the waveguide. It is the general belief in the art that in order for electrons to interact strongly with the electromagnetic field of the wave to be amplified, these electrons must possess an energy far in excess of 100 keV (see U.S. Pat. No. 3,457,450 noted above) and perhaps in the MeV range as in the experiment by Destler et al. The reason for this perception in the art of a need for a high energy relativistic electron beam is that only with such a beam would the Larmor radii of the electrons be sufficiently large to couple strongly with the Te.sub.L1 mode. However, placing a energetic electron beam very close to the waveguide wall for the device in order to couple with the TE.sub.L1 mode is not an attractive feature because of the potential for waveguide burnout if the beam is even slightly misaligned. Additionally, operation at a high cyclotron harmonic via the propagation of a highly relativistic electron beam inside an unloaded waveguide leads to serious problems in mode competition. Finally, and most importantly the generation of such a highly energetic electron beam in the MeV range would require a device with a volume on the order of a small room. Thus, such a highly relativistic electron beam is simply not practical for standard millimeter wave device fabrication. Accordingly, it can be seen that there are significant drawbacks to the use of high cyclotron harmonic frequencies sufficient to allow the elimination of the superconducting magnet requirement.